X-ray tubes require a source of power to a filament in order to produce the electrons which will be accelerated to produce X-rays. In most X-ray tubes, the filament is part of the cathode structure of the tube, which is several to many tens of kilovolts (kV) negative in potential with respect to ground potential. Because control electronics and the prime power source are generally located at ground potential, a means of delivering power on the order of tens of watts across this potential barrier is needed.
Historically, this means of delivering power consists of some sort of transformer whose primary winding is at ground potential and whose secondary winding is sufficiently insulated to withstand the cathode high voltage potential reliably even in the face of arcing either in the X-ray tube, or in the high voltage power generator itself, or in the cable connecting them. A safe and reliable means of constructing such a transformer involves winding a primary around one leg of a four sided core. The secondary winding is generally contained within a circular tube concentric with the primary winding. The opening in the core through which the windings are wound is called the window.
To maintain reliable isolation, such a transformer must have large separations between the secondary and primary windings. This requires a very large window, and by extension, a very large core. The size of the transformer can be reduced by operating it in an insulating medium, such as oil, pressurized gas, or a solid potting material. However, the size of the transformer is still quite large for typical operating voltages of 100 to 250 kV. Attempts to reduce the size of such a transformer inevitably reduce the insulation reliability since the entire voltage is “held off” by one insulating space.